The present invention relates to organic electroluminescent devices which comprises a mixture of at least two materials, one of which is a hole-transport material, in at least one hole-transport layer.
The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136. However, there continues to be a need for improvement, in particular with respect to the efficiency and lifetime. A further problem for which there is still a need for improvement is the so-called “roll-off” behaviour. This is taken to mean that the efficiency of an organic electroluminescent device is usually significantly lower at high luminous density than at low luminous density. Thus, very high efficiency is obtained at very low luminous densities, while only low efficiency is obtained at high luminous densities, in particular in the case of phosphorescent OLEDs. The very high efficiency at low luminous density is therefore undesired since this is accompanied by problems in the addressing of the OLED. By contrast, a higher efficiency at high luminous density would result in higher power efficiency and thus in lower energy consumption of the OLED. Further improvements in the roll-off behaviour are therefore desirable.
Furthermore, there is still a need for improvement in the processability of the materials. Thus, there are in some cases problems that the hole-transport materials in particular crystallise, for example on the shadow mask, during application, which makes device production significantly more difficult. This is particularly problematic, in particular, during application of the hole-transport layer, since these layers are usually very thick, meaning that a lot of material is used here. Crystallisation of the materials thus has particularly adverse effects here.
The technical problem on which the present invention is based is therefore the provision of an organic electroluminescent device which exhibits higher power efficiency and/or a longer lifetime and/or an improved roll-off behaviour and/or which has an improvement in the processing of the layers during production.